Waveguide window



April 14, 1959 M. M. FREUNDLICH 2,882502 WAVEGUIDE wmoow Filed April 19. 1954 FIG. 20

FIG. 3

INVENTOR. MARTIN u. v=nsuum.ncu

ay fi eiz......qwa.g3umf ATTORNEYS United Statos Patefif WAVEGUIDE WINDOW Martin M. Freundlich, Huntington Station, N.Y., as-

signor, by mesne assignments, to Cutler-Hammer, Inc. Milwaukee, Wis. a corporation of Delaware Application April 19, 1954, Serial N0. 423,893 k 11 Claims. (C1. 333-98) This invention relates to waveguide windows and a method of manufacture thereof, and is especially directed to the provision of a low-loss wideband window suitable for use over a substantial frequency range.

Waveguides are in common use for the transmission of ultra-high-frequency radio energy. For example, wave-guides are in widespread use for the transmission of energy in the range of 3,000 megacycles per second (mc.) and are sometimes employed at lower frequencies. At still higher frequencies, such as 10,000 mc. and above, waveguides are use d almost exclusively for transmiss1on.

Usually the cross-sectional dimensions of the waveguide are selected to be smaller as the frequency of operation increases. The low frequency cutolt 1s netermmea largely by the dimensions selected, and the usefulness of a given waveguide is greatly impaired by spurious modes of transmission when the frequency exceeds some upper limit. Thus a waveguide of given cross-sectional dimensions is commonly employed only for a limited frequency range. For practical reasons, the cross-sectional dimensions of waveguides have been considerably standardized in eaoh frequency. range. For exarnple, in the region of 000 mc. a type of waveguide designated RG-52/U is often employed. This is a rectangular waveguide whose internal cross-sectional dimensions are approximately 0.90 x 0.40 inch and is employed to transmit frequencies in the range of about 8,200 to 12,400 mc. Other .standard waveguides are available forother frequency ranges. Although waveguide of rectangular cross-section is most widely employed, circular waveguide is also usedupou occasion, and the present invention is applicable to either type.

I t is often desirable to place a window in a waveguide so as to permit maintaining a vacuum or pressure yielding a relatively low loss and low SWR at that frequency. While often satisfactory for a given applica; tion, such resonant structures inherently have a relatively narrow band, and the losses and standing wave ratio increase fairly rapidly outside of that band.

Glass has commonly been employed as the window material. However{ even so-called low-loss glass in practical thicknesses introduces suflicient attenuation and dielectric discontinuity in a waveguide as to confine its usefulness largely to resonant type structures. lt is also known that high-grade mica in thin sheets has very low losses at ultra-high frequencies, and some success has been attained with mica in windows of the resonan type. 2

It is a primary object of the present invention to pro: vide a low-loss wideband waveguide window, and to this end mica is employed as the window material in a non resonant structure. It is also an object of the invention to provide a window of this type which can be inserted readily in a waveguide. Other objects and advantages of the invention will in part be pointed out and in part be evident from the description given hereinafter.

In accordance with the invention, a thin sheet of mica is sealed between a pair of aligned metal frames having inside dimensions equal to the internal crosssectional dimensions of the waveguide. The sheet of mica has planar dimensions greater than the inside di mensions of the frames and is sealed between the frames by a cementing material, preferably glass, which extends between the opposed surfaces of the frames so as to form a strong vacuum-tight seal. The cementing material advantageou'sly extends substantially to, but not beyond, the inside edge;s of the frames. Particularly in the case of glass, this has been found to be of considerable importance in order .to provide a streng seal and yet avoid losses and increased SWR. The thickness of the mica and cement layer is very sinall compared to a wavelength over the frequency range of the waveguide. 'AS a result, it is found that when the window is soldered into a waveguide, the losses and SWR are very low over a wide range of frequencies.

The invention Will be more fully understood by reference to the following detailed description thereof,iaken in conjunction with the drawings, in which:

Fig. 1 is a perspective view of one specific window 'of the invention;

in one part of the waveguide with respect to another, or to exclude rain .and moisture, etc. It is important that such a window have low losses so as to attenuate fchg: t1ansmitted energy as little as possible, and also to avoid s etting up standing waves which might eflFect ad- Yersely the erformance of equipmnt with Which the 4- waveguide is used. -It is also highly desirable to have a. window which will transmit nergy over a wide frequency range With low attenuation and low standing wave ratio (SWR), preferably over the whole frequency range of the waveguide With which it is used, thus per mitting use in applications where the ttansmission or reception of signals over a wide frequency range is requir ed, and avoiding the need of producing and stocking a large number of narrow band windows for spot iroqnency applications.

- There have been many proposals for constructing Waveguide windows, and some have been used in pracfi :e. Most commercial windows have been of the raonant type, in which the area and thickness of the iv indow and the designof the associated mounting are choSei1 so as to resonate at a certain frequency, thus Fig. 2 is a cross-section taken along the line 22 of Fig. 1;

Fig. 2a is a detail illustrating an undersirable formation of the glass sealing layer;

Fig. 3 illustrates the window soldered in waveguide; and

Fig. 3a is a view of one portion of the waveguide -of Fig. 3, showing details of the flange recess.

Referring now to Fig. l, a waveguide window is shown comprising a pair of aligned thin metal frames 10, 11, with a thin sheet of'mica 12 sandwiched therebetween. The mica sheet is sealed between the metal frames by a cementing material, advantageously glass, which also joins the two metal frames 10, 11 so as to form a unitary structure.

The window of Fig. 1 is designed for use with a rectangular waveguide and the inner dimensions 13, 14 of the frames, that is, the dimensions of the window opening, axe made equal to the internal cross-sectional di mensions of the waveguide with which it is to be employed.

Fig. 2 is a cross-section of the window of Fig. 1 and shows the metal frames 10, 11 sealed in alignment by a thin fused layer of glass 15. The mica sheet. 12 has planar dimensions which are greater than the inside dimensions of the frames' so that it overlaps the frames,

a section of ais shown at 12'. Thus, the glass layer 15 joins the mica sheet and the metal frames in a. single unitaxy construc tion which is vacuum-tight.

Advantageously, as shown in Fig. 2, the planar dimensions 013 the mica sheet 12 are less than the outside dimensions of the frames, so that the fused layer of glass 15 extends between the opposed surfaces of the frames on both sides of the mica sheet and also around the outer edge thereof. Thus the mica sheet is firmly bonded to both metal frames, and the outer portions of the frames are directly bonded together by the fused glass. In eflect, the mica sheet is embedded in the glass layer. The result is a very streng vacuum-tight joint.

The thickness f the mica sheet 12 and glass layer 15 is made very small compared to a wavlength over the frequency range of operation in a wavegnide. Thus, when the window is inserted ina waveguide, the metal frames become part cf the conducting waveguide wall and introduce no substantial discontinuity. The mica and glass layer does introduce a discontinuity, but the discontinuity is very short and consequently the losses and reflections caused thereby arevery small.

It is desirable to select the metal -of the frames and the glass so as to match the thermal coefficient of ex pansion of the mica as nearly as possible. Glass is preferred as the cementing material since it forms a streng permanent air-tight seal, and also will withstand ordinary soldering temperatures cf, say, 250 C. 01' thereabouts. This is an important advantage, since soldering the metal frame into a waveiguide insures good electrical contact to minimize the length of the discontinuity in the waveguide wall. However for some purposes lt may be found satisfactory to employ other cementing materials, such as plastic cements, etc.

In a particular window which has been used with success, the metal frames were punched out of Allegheny- Ludlum Alloy N0. 4750 or Driver-Harris Alloy N0. 52. These alloys are understood to contain approximately 50% nickel and 50% iron. The glass was Corning Type 7570, which is sometimes termed a solderf glass. The window was specifically designed for use with waveguide type RG-52/U, mentioned above, and the internal dimensions 13 and 14 were 0.400 inch and 0.900 inch, respectivley, to match the internal dimensions of the waveguide. The external dimensions of the window were .597 x 1.097 inches, and the thickness of each frame was .030 inch. A thin sheet of mica approximately 0.002 inch thick was employed, and the thickness of the glass layer 15 was approximately 0.006 inch, giving an overall thickness of the win dow of approximately 0.066 inch.

This window was found to have an average SWR cf approximately 1.05 over the frequency range from 8200 to 12,400 rnc. In a typical lot, the maximum standing wave ratio over this frequency range for the poorest window was 1.08. Taking a frequency near the middle of this range, 10,000 mc. the wavelength in air is 3 cm. (1.2 inches) and only slightly difierent in the waveguide. Thus the thickness of the mica-glass layer (0.006 inch) is very small compared to the wavelength.

In producing this window, the metal frarnes were first degreased, chemically cleaned and hydrogen-fired. The frames were then coated with a glass frit composed of the above-mentioned Corning glass mixed with water and heated in air to approximately 590 C. After cooling, the glass layer onthe frames was ground to a thickness cf approximately 0.003 inch.

The mica was then assembled between the frames and the assembly aligned in a jig. To facilitate handling and preeise assembling, the mica was glued to the lower frame with a cellulose binder, and the upper fran1e then glued in place with the same binden Small, weights were placed on top of the assembly and the unit sealed together by baking at a temperatu re of about 600 C. for about -15 minutes to fuse the; glass. The glueing step is advantageous to secure proper alignment, and does not appear to afiect adversely the final seal since the sealing temperature is sulficiently high to burn out the bindet.

This process resulted in a streng vacuum-tight joint. However, the sealing procedure left a fillet of glass 15' at the inside edges of the metal frames of the nature illustrated in Fig. 2a. This glass fillet is highly undesirahle since it considerably increases the SWR of the window. In rectangular waveguides of the type illustrated, it was found that if the fillet extended a few thousandths of an inch into the winclow area, on the long edges of the rectangle, there was a marked increase in SWR. The fillets on the narrow edges weie not so objectionable due to the fact that the direction of the electrical field in rectangular waveguides is commonly perpendicular to the longer side. However, though of less er importance, an excessive glass fillet 011 the narrow edges is undesirable. In the case of wund waveguides, it is considered that the presence of the glass fillet at any point around the circumference of the window area is undesirable. If the glass layer recedes from the jnner edges of the frames, the electrical properties of the window will not be afiected materially, but the strength 0f the seal will be weakened and in many cases such weakening will be undesirable.

In Order to eliminate the glass fillet, a glass is advantageously selected which, when contacted by an acid solution of suitahle composition, is attacked by the acid so as to decompose the glass or destroy its vitreous nature, with resultant disintegration of the fillet. The fillet may then be removed by a small soft scraper, such as a pointed wooden stick.

The composition of the glass should be ehosen with respect to that of the acid solution so that the glass can be disintegrated, as described, while at the same time the acid solution must be such as not to harrn the metal frames. F01 the glass mentioned above, a first cleaning operation was efiected by dipping the windows for lwvo minutes in a bath made of 24 oz. sulphuric acid (commercial grade) in a gallon of water, at C. This loosened the scale on the metal frarnes. The frames were then placed for two minutes at room temperature in a bath made as follows:

pint sulphuric acid (commercial grade) fi pint hydrofluoric acid (commexcial grade) 8 oz. chronn'c acid Water to make one gallon.

This treatment removed the scale on the metal frames and also changed the glass of the fillet to a white powder which could be removed mechanically as described above. The depth to which the glass is attacked increases with duration cf immersion, so that the duration can be selected with respect to the size of the glass fillet to be disintegrated and removed. Any glass remaining 0n the outside edge of the window may be ground off.

The frames were then silver-plated so as to insme making good contact with a waveguide. This was accomplished by cleaning the windows in a 50% solution by volume cf nitric acid, silver-plating the windows, rinsing in running water, rinsing in acetone, and drying in a strearn cf air.

Referring now to Fig. 3, the window is shown inserted in a flange 16, 17 of a rectangular waveguide 18. The window made as described is capable of withstanding a considerable temperature, well above the temperature required for efiective soldering. While the design of the flange is subject to considerable variation, dependi.ng upon the use contemplated, One design is shown in Fig. 3a. Here one section of waveg uide 18 terminates in anv nutwardly extending flange 16 having a recess 21 whose dimensions accommodate the window.

One soldering technique which can be employed with this construction is tov pre-tin the internal .surfaqes oi Ihn -r.eCess -2-1;1and apply a amounfofinon-emmsi ve Soldat, pastd to .the window frame .edge, and undat- 8id.e-w-;The: waveguide is held vertically, =the window placed in the rec ess area, and the flange heated from the underside until the window drops in place and seats. Advantageously the depth of the recess 21 is substantially equ.al to.. the thickness of the frame of the window, so that the frame is flush with the surface of the flange sahen seated therein. Gare should cf course betaken to avoid. any solder runm'ng onto the mica window area Dr in to adjacent portions of the waveguide opening. The matching section of the waveguide may be provided with a fl at flange '17 whichcan ths be bolted to flange 16 in y;vhich. the window is se'ated, or secured the'reto. in any su itable manner.- =;I't will be apparent'that many details of tl1'e specific embodiment described may be changed within the: spirit and scope of the invention. Also, the numerical values given in t he example clescribed herein -may be changed as required fpr a particular application, these values being given mexely as an aid to the ready practice of the invention. .-Ihe shape and.overall dimensions .willof caurse be altered fordifl?erent sizes and shapes Cf vvavegni'des. While the method ofmaking the window described herein has been found satisfactory in pra'ctice, any other suitable method of making it may of course be employed if desired.

I claim:

1. A low-loss wideband window for an electromagnetic waveguide which consists cf a pair of aligned metal frames cf predetermined inside and outside dimensions, a thin sheet of mica having planar dimensions greater than said inside dimensions but lass than the outside dimensions cf said frames, said thin sheet cf mica being substantially transparent to ultra-high-frequency electromagnetic energy, and a cementing layer sealing said mica sheet between said frames, said cementing layer extending substantially continuously around the border of each side of the mica sheet between the mica sheet and the opposed surface of the respective frame and substantially continuously outside the edges cf the mica sheet between the opposed surfaces of the frames, said cementing layer firmly uniting said frames and firmly nniting each side cf the mica sheet to the respective frame.

2. A low-loss wideband window for an electromagnetic waveguide which consists of a pair of aligned metal frames of predetermined inside and outside dimensions, a thin sheet cf mica having planar dimensions greater than said inside dimensions but less than the outside dimensions of said frames, said thin sheet of mica bex'ng substantially transparent to ultra-higbfrequency electromagnetic energy, and a cementing layer sealing said mica sheet between said frames, said cementing layer extending substantially continuously around the border of each side of the mica sheet between the mica sheet and the opposed surface cf the respective frame and substantially continuously outside the edges cf the mica sheet between the opposed surfaces of the frames, said cementing layer extending towards but not beyond the inside cdges cf the said frames and firmly uniting said frames and firmly uniting each side cf the mica sheet to the respective frame.

3. A low-loss wideband window for an electromagnetic waveguide which comprises a pair of aligned metal frames of predetermined inside and outside dimensions, a sheet of mica havingplanar dimensions greater than said inside dimensions but lass than the outside dimensions of said frames, said mica sheet being sealed between said frames by a fused glass layer joining both sides of the mica sheet near the e'adge thereof to the respectively opposed surfaces cf the frames and joining together the opposed surfaces cf the frames around the edge of the mica sheet, said glass layer extending substantially to but not beyond the inside edges of said frames.

4. A low-loss wideband window for an electromagnetic wa%gtiidni wliidh comprisesm: pair Iof* a1igncdllmetal frames havingrec'tangularwinduw openings ofpredeter mined dimensions, a sheet cf mica having planar dimensions greater than said window opening dimension's but 1ess than the ou'tside dimensionS of said frames, said mica sheet' being sealed between said frames bya fused glass layer joining both sides of the micasheet near the edge thereof to the respectively opposed surfaces of the frames and joining together the opposed surfaces of thc': frames around the edge of the mica sheet. said glass layer extending towards but not beyond the longer edg'es of said rectangular window openings of the frames.

5. A low-loss wideband window for insertion in an electromagnetic waveguide cf predetermined inside cross- 'sectional dimensions which conxprises a pair cf aligned metal frames having.inside dimensions equal to the inside cross-sectional dimensionszof said waveguide, a sl 1eet of. mica having planar dimensions greater than said inside dimensions but lass than theoutside dimensions of said frames, said mica: sheet being sealed between said frames by a cementing layer joiningboth sides of the mica 'sheet near the edge thereof to the r6spebtivelY opposed surfaces of the fraimes and joining together the .opposed surfaces Of'1h2 frunes around the edge of th mica sheet. 1;

6. A low-loss wideband window for insertion in an electromagnetic waveguide 015 predetermined inside crosssectional dimensions adapted to transmit ultra-high-frequency radio energy over a substantial frequency range which comprises a pair of aligned metal frames having inside dimensions equal to the inside cross-sectional dimensions cf said waveguide, a sheet of mica having planar dimensions greater than said inside dimensions but less than the outside dimensions of said frames, said mica sheet being sealed between said frames by a fused glass layer joining both sides of the mica sheet near the edge thereof to the respectively opposed surfaces of the frames and joining together the opposed surfaces of the frames around the edge of the mica sheet, the thickness of the mica sheet and glass layer being very small compared to a wavelength over said frequency range.

7. A low-loss wideband window for insertion in an electromagnetic waveguide of predetermined inside crosssectional dimensions adapted to transmit ultra-high-frequency radio energy over a substantial frequency range which comprises a pair of aligned metal frames having rectangular window openings of dimensions equal to the inside cross-sectional dimensions of said waveguide, a sheet of mica having planar dimensions greater than said window opening dimensions but lass than the outside dimensions of said frames, said mica sheet being sealed between said frames by a fused glass layer joining both sides of the mica sheet near the edge thereof to the respectively opposed surfaces of the frames and joining together the opposed surfaces cf the frama around the edge of the mica sheet, said glass layer extending substantially to but not beyond the langer edges of said rectangular window openiimgs of the frames, the thickness of the mica sheet and glass layer being very small compared to a wavelength over said frequenc) range.

8. A low-loss wideband window for insertion in an electromagnetic waveguide of predetermined inside cross sectional dimensions adapted to' transrnit ultra-high-frequency radio energy over a. substantial frequency range which comprises a pair of aligned metal frames having inside dimensions equal to the inside cross-sectional dimensions of said waveguide, a sheet of mica having planar dimensions greater than said inside dimensions but less than the outside dimensions of said frames, said mica sheet being sealed bt:tween said frames by a fused glass layer joining both sides of the mica sheet near th edge thereof to the respectively opposed surfaces of the frames and joining together the opposed surfaces of the frames around the edge of the mica sheet, said glass layer 'oxtendix'1g srflbstantially to.but uot bcyond the. insicle edges cf said frames, the thickr'xessnf the mica sheet and glass Iayer' being very small:i:mixpared to a wavelength ovex' said frequency range.

1 9. A-method of making a.low-loss window foxan e1ectromagnetic. wavegx'1ide which comprises sealing a sheet of'mica between a p1ir:of aligned metal frames by a fused glass layer, hc at and pressure being applied during said sealing whereby a fillet of glass is formed inside an inner edgc cf said frames, contacting said fillet with an acid solution s9lectecl to disintegrate the glass cf said fillet, and nmoving said disintegrated fillet.

10. A method of making a low-loss window for.an elcctromagnetic. waveguide .which comprises fusing a la.yer of glass to one side.of each of a pair cf metal frames, assembling. asheet of mica between said frames in contadt with thmglass layer, heating the assembly under pressure. to fu sethc': glass layers a.nd thereby form a sealing laYer extending, between the opposed sixrfaces of theframes an both sides cf the mica sheet, said beating a.nd pressure re sulting in the=iormation of a fillet of glass insidc an inner edge of said frames, contacting the assembly with an acid solution selected with respect to said glass to disintzgrate said fillet, and removing said disintegrated fillet.

11 A niethod o1! making a l09idoss' window: for=1'n electromagnctic waveguide whichcomprises fusing a lag! er of glass to one side of each cf a 5air of metal framls; grinding the glass lay'er on each frame to a subsiahtiall? uniform thickness; assembliflg a 'sheet ofmica betwcen sziid fiames in contact withihe glass layers, the plamir dimensions of the mica sheet being greater thau the inside dimensions cf said frames but lass than the outside dimexi sions thereof, heatingthe assembly under pressre toftl56 the glass 1ayers and thereby form a sealing layencxten& ing betweeu the opposed surfaces df the frames on both sides cf the mica sheet and around the edge theredf-said heating and pressure resulting in the formation of a. flllet of glass inside an inner edge of said frames, immersing the assembly in an acid solution selectedwith respect to said glass to' disintegratasaid fillet, and me'x:hanically removing saiddisintegrated fillet. 1 1

-Reference Cited in the fileof this patent M UNITED1STFATES PATENTS 219s7a9 Goodalc ............"Apr. so, 1940 2,453645 Tiley Nov. 9, 1948 2,ssss1 McArthur mm 12, 1951 2,6369 26 y Wilson -......-.m-.. Apr. 28, 1953 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 2,88202 April l4, l959 Martin 1VL Freundlich r appears in the printed specification It is hereby certif-ied that erro that the said Letters of the above numbered patent requiring correction and Patent.should read as corrected below.

Column 6, line 55, for frame" re'ad frames Signed and sealed this 28th day of July l959 "SEAL) ttest:

ROBERT C. WATSON KARL H. AXLINE Commissioner of Patents Attesting Officer 

